Protein kinases are families of enzymes that catalyze the phosphorylation of specific residues in proteins, broadly classified in tyrosine and serine/threonine kinases. Inappropriate activity arising from dysregulation of certain kinases by a variety of mechanisms is believed to underlie the causes of many diseases, including but not limited to, cancer, cardiovascular diseases, allergies, asthma, respiratory diseases, autoimmune diseases, inflammatory diseases, bone diseases, metabolic disorders, and neurological and neurodegenerative diseases. As such, potent and selective inhibitors of kinases are sought as potential treatments for a variety of human diseases.
There is considerable interest in targeting the innate immune system in the treatment of autoimmune diseases and sterile inflammation. Receptors of the innate immune system provide the first line of defense against bacterial and viral insults. These receptors recognize bacterial and viral products as well as pro-inflammatory cytokines and thereby initiate a signaling cascade that ultimately results in the up-regulation of inflammatory cytokines such as TNFα, IL6, and interferons. Recently it has become apparent that self-generated ligands such as nucleic acids and products of inflammation such as high-mobility group protein B1 (HMGB1) and Advanced Glycated End-products (AGE) are ligands for Toll-like receptors (TLRs) which are key receptors of the innate immune system (O'Neill 2003, Kanzler et al 2007, Wagner 2006). This demonstrates the role of TLRs in the initiation and perpetuation of inflammation due to autoimmunity.
Interleukin-1 receptor associated kinase 4 (IRAK4) is a ubiquitously expressed serine/threonine kinase involved in the regulation of innate immunity (Suzuki & Saito 2006). IRAK4 is responsible for initiating signaling from TLRs and members of the IL-1/18 receptor family. Kinase-inactive knock-ins and targeted deletions of IRAK4 in mice were reported to cause reductions in TLR and IL-1 induced pro-inflammatory cytokines (Kawagoe et al 2007; Fraczek et al. 2008; Kim et al. 2007). IRAK4 kinase-dead knock-in mice have also been shown to be resistant to induced joint inflammation in the antigen-induced-arthritis (AIA) and serum transfer-induced (K/BxN) arthritis models (Koziczak-Holbro 2009). Likewise, humans deficient in IRAK4 also appear to display the inability to respond to challenge by Toll ligands and IL-1 (Hernandez & Bastian 2006). However, the immunodeficient phenotype of IRAK4-null individuals is narrowly restricted to challenge by gram positive bacteria, but not gram negative bacteria, viruses or fungi. This gram positive sensitivity also lessens with age, implying redundant or compensating mechanisms for innate immunity in the absence of IRAK4 (Lavine et al 2007).
These data indicate that inhibitors of IRAK4 kinase activity should have therapeutic value in treating cytokine driven autoimmune diseases while having minimal immunosuppressive side effects. Additional recent studies suggest that targeting IRAK4 may be useful in other inflammatory pathologies such as atherosclerosis and diffuse large B-cell lymphoma (Rekhter et al 2008; Ngo et al 2011). Therefore, inhibitors of IRAK4 kinase activity are potential therapeutics for a wide variety of diseases including but not limited to autoimmunity, inflammation, cardiovascular diseases, cancer, and metabolic diseases. See the following references for additional information: N. Suzuki and T. Saito, Trends in Immunology, 2006, 27, 566. T. Kawagoe, S. Sato, A. Jung, M. Yamamoto, K. Matsui, H. Kato, S. Uematsu, O. Takeuchi and S. Akira, Journal of Experimental Medicine, 2007, 204, 1013. J. Fraczek, T. W. Kim, H. Xiao, J. Yao, Q. Wen, Y. Li, J.-L. Casanova, J. Pryjma and X. Li, Journal of Biological Chemistry, 2008, 283, 31697. T. W. Kim, K. Staschke, K. Bulek, J. Yao, K. Peters, K.-H. Oh, Y. Vandenburg, H. Xiao, W. Qian, T. Hamilton, B. Min, G. Sen, R. Gilmour and X. Li, Journal of Experimental Medicine, 2007, 204, 1025. M. Koziczak-Holbro, A. Littlewood-Evans, B. Pollinger, J. Kovarik, J. Dawson, G. Zenke, C. Burkhart, M. Muller and H. Gram, Arthritis & Rheumatism, 2009, 60, 1661. M. Hernandez and J. F. Bastian, Current Allergy and Asthma Reports, 2006, 6, 468. E. Lavine, R. Somech, J. Y. Zhang, A. Puel, X. Bossuyt, C. Picard, J. L. Casanova and C. M. Roifman, Journal of Allergy and Clinical Immunology, 2007, 120, 948. M. Rekhter, K. Staschke, T. Estridge, P. Rutherford, N. Jackson, D. Gifford-Moore, P. Foxworthy, C. Reidy, X.-d. Huang, M. Kalbfleisch, K. Hui, M.-S. Kuo, R. Gilmour and C. J. Vlahos, Biochemical and Biophysical Research Communications, 2008, 367, 642. O'Neill, L. A. (2003). “Therapeutic targeting of Toll-like receptors for inflammatory and infectious diseases.” Curr Opin Pharmacol 3(4): 396. Kanzler, H et al. (2007) “Therapeutic targeting of innate immunity with toll-like receptor agonists and antagonists.” Nature Medicine 13:552. Wagner, H. (2006) “Endogenous TLR ligands and autoimmunity” Advances in Immunol 91: 159. Ngo, V. N. et al. (2011) “Oncogenically active MyD88 mutations in human lymphoma” Nature 470: 115.